The present invention relates to a process for operating a steam thermal engine and a device for operating a steam thermal engine. In particular, the invention relates to a steam engine, which can be operated with waste heat, especially from a burner or a combustion engine.
The DE 196 10 382 A1 discloses a steam engine, whose working medium is converted into superheated steam in an exhaust gas heat exchanger. The exhaust gas heat exchanger obtains its energy from the lost energy or from the waste heat of a combustion engine, which is coupled with the steam engine. To this end, the coolant and exhaust gas of the combustion engine pass through the exhaust gas heat exchanger.
Consequently, in this system an attempt is made to utilize the lost heat of a combustion engine in order to increase the total efficiency of the system. Under optimal operating conditions, one-third of the energy supplied in the form of fuel is converted into mechanical energy by a combustion engine, one-third is dissipated by way of the cooling water, and the other one-third is dissipated by way of the hot exhaust gas.
The use of the waste heat of a combustion engine is complex because there are two media as the transfer agents of the lost energy, which are the cooling water and the exhaust gas. The energy, present in both media, can be fed only inadequately to a single system for energy recovery or energy conversion, because their respective thermodynamic parameters are different. The temperature level of the cooling water is approximately 100xc2x0 C. In contrast, the temperature of the exhaust gas is approximately 300xc2x0 C. at operating points with a low load up to 900xc2x0 C. at operating points with a high load.
If, for example, water is used as the medium in the circulation of the thermal engine, the result is a boiling temperature of 100xc2x0 C. at a pressure of 1 bar. To realize a desired pressure level of 10 bar, however, a temperature of 180xc2x0 C. is necessary. This eliminates the cooling water of the combustion engine as the energy supplier, because the cooling water temperature would have to reach a minimum of 200xc2x0 C. to guarantee the requisite energy transfer by a heat exchanger.
Another problem is that the design of the entire system must be simple in order to achieve a low weight so that use in the automotive field is possible.
The present invention provides a solution to the problem by providing a thermal engine that enables a fast and efficient conversion of thermal energy, which is stored in media and has varying thermodynamic parameters, into mechanical energy.
According to the present invention, the thermal energy from the media with different thermodynamic parameters can be converted efficiently into a hot steam, which in turn can be converted efficiently into mechanical energy in the expansion unit, by taking the following steps: heating the working medium of a thermal engine in a boiler to a low temperature of preferably boiling temperature at low pressure; feeding steam from the boiler into a pressure vessel, in which the steam is heated to a high temperature; injecting the liquid working medium (or condensate) from the boiler into the pressure vessel, whereby the working medium is instantaneously evaporated, whereby the pressure in the pressure vessel increases rapidly; and by feeding the hot steam from the pressure vessel to an expansion unit.
Injecting the boiling working medium into the pressure vessel, which is heated with, for example, hot exhaust gas, and in which there is already a certain amount of hot steam, enables a spontaneous and enormously large pressure increase in the pressure vessel. The increase in pressure can be passed onto the expansion unit.
In this manner the boiler can be heated especially advantageously with the cooling water of a combustion engine and the pressure vessel can be heated with the exhaust gas stream of the combustion engine, whereby a high pressure level can be generated in the circulation of the thermal engine. Alternatively, the boiler is heated with the steam, which has already been expanded by the expansion unit, and the pressure vessel is heated by a burner.
In an especially advantageous design the steam from the boiler is pre-compressed and then fed to the pressure vessel, whereby both the pressure and the temperature of the steam in the pressure vessel increase before the liquid working medium is injected into the pressure vessel. In this manner the pressure level is also increased in the pressure vessel with the temperature increase.
In an especially advantageous design, not only the boiler is heated by the hot coolant of a combustion engine and/or by the steam, which has already been expanded in the expansion unit, and the pressure vessel is heated by the exhaust gas of the combustion engine, but also the exhaust gases are even used to heat the boiler, working on a low temperature level, after the pressure vessel has been heated up. Thus, the waste heat from a combustion engine can be used more completely for the expansion unit.
By coupling the shaft of a combustion engine, whose waste heat is recovered for the expansion unit, with the shaft of the expansion unit with a coupling and/or with a reduction gear, the expansion unit and the combustion engine can be joined together in a simple manner when adjusting the different speed levels or decoupled from each other.
In an especially advantageous design, at least two cylinders of a piston machine are provided with one allocated pressure vessel each as the expansion unit, whereby a reciprocating motion of the piston machine can be used to pre-compress the steam coming from the boiler; and a subsequent reciprocating motion of the piston can be used to expand the high pressure steam from the pressure vessel. In so doing, it is possible to allow the steam generating process in a pressure vessel and its expansion in one of the expansion units to take place alternatingly, for example, between the two cylinder units in such a manner that enough time remains for the respective steam conditioning in the respective pressure vessel.
Three preferred embodiments are described in detail below in the following with reference to the attached drawings.